基于光电相位调制器的微波毫米波段DPASKQAM光信号的产生

2332OPTICSLETTERS/Vol.33,No.20/October15,2008

PhotonicDPASK/QAMsignalgenerationatmicrowave/millimeter-wavebandbasedonan

electro-opticphasemodulator

YeZhang,*KunXu,RanZhu,JianqiangLi,JianWu,XiaobinHong,andJintongLin

KeyLaboratoryofOpticalCommunicationandLightwaveTechnologies,MinistryofEducation,BeijingUniversityof

PostsandTelecommunications,Beijing100876,China*Correspondingauthor:zhangye2003@http://wendang.chazidian.comReceivedJune17,2008;revisedAugust21,2008;acceptedAugust21,2008;postedSeptember5,2008(Doc.ID97556);publishedOctober10,2008

Wehaveproposedandexperimentallydemonstratedtwonovelphotonicarchitecturestogeneratedifferential-phaseamplitude-shiftkeyingandcircularquadratureamplitudemodulationsignalsatmicrowave/millimeter-wavebandbasedonanelectro-opticphasemodulator.Inourproposedschemes,theelectronicdrivencircuitsweregreatlysimpli?edbyemployingthephotonicvectormodulationtechnique.©2008OpticalSocietyofAmerica

OCIScodes:060.4080,060.5625,350.4010.

Recently,theradio-over-?ber(RoF)techniquehasre-ceivedmuchattentionasasolutioninrealizingfu-turebroadbandaccessnetworksinhotspotsandsub-urbanareasowingtowell-knownadvantages,suchaslowtransmissionloss,hugeavailablebandwidth,avoidanceoffrequencycongestion,andsoon.Insuch?berwirelessnetworks,manyef?cientmodulationformats,includingamplitude-shiftkeying(ASK),phase-shiftkeying(PSK),frequency-shiftkeying(FSK),andquadratureamplitudemodulation(QAM),etc.,havereceivedmuchattentiontoincreasespectralef?ciency.Amongtheseef?cientmodulationformats,differential-phaseamplitude-shiftkeying(DPASK)andcircularQAM,effectivelymultiplexingdifferential-phaseencoding,andamplitudemodula-tionareattractiveforhavingalowerpeak-to-mean-envelope-powerratiocomparedwithASKandrectan-gularQAM[1–3].However,thetraditionalelectricalgenerationofsuchmultilevelsignalsatmicrowave/millimeter-wave(mm-wave)bandmaygreatlyin-creasecomplexityandcostin?berwirelesssystems.Therefore,oneofthekeytechniquesisthephotonicgenerationofmm-wavesignalsbyemployingthephotonicvectormodulation(PVM)techniquetosim-plifytheelectronicdrivencircuitinthetransmitterside[4].InthisLetter,basedonourpreviousstudyonelectro-opticphasemodulator(EOPM)[5,6],weproposetwoPVMarchitecturestogenerateDPASKandacircularQAMsignalatmillimeter-wavebandasanewchoiceforfuturemillimeter-wavesignalgeneration.Aproof-of-conceptexperimentiscon-ductedbyemployingtwo2Gbits/spseudorandombi-narysequences(PRBSs)atacarrierfrequencyof25GHz.ThestudyoftransmissionperformanceisalsoreportedinthisLetter.

Theschematicprincipalcon?gurationofthepro-posedDPASKPVMsystemisdepictedinFig.1.Thecwfromalasersourceis?rstmodulatedbytheRFsinusoidalclockviaadual-armLiNbO3Mach–Zehndermodulator(MZM)atitsdouble-sidebandcarrier-suppression(DSB-CS)modetogenerateanopticalmmwave.Thetwosidebandsareseparated

0146-9592/08/202332-3/$15.00

byanopticalinterleaverandinjectedintotheEOPMwithoppositedirections.ThustheuppersidebandpassesthroughtheEOPMwithapositivedirectionwhilethelowersidebandpasseswithanegativedi-rection.TheEOPMislocatedatthecenteroftheloopanddrivenbythephaseinformation.Aftertheopti-calcirculator,thetwoopticalsidebandsarecombinedbyanopticalcoupler(OC)andinjectedintoMZM2modulatedbytheamplitudesignalserial.Thentheelectric?eldoftheoutputopticalsignalcanbegen-erallyexpressedas

E?t??A?t??ELDexpj???LD??RF?t+?c?t??

+ELDexpj???LD+?RF?t+?s?t???,

?1?

whereELDistheelectric?eldofthecwfromthelasersource,A?t?representstheamplitudeinformation,?cand?sarethephaseshiftofthelowersidebandandtheuppersideband,respectively,and?LDand?RFaretheangularfrequencyofopticalcarrierandRFclock.SincetheMZMisanopticalpolarizationde-pendentdevice,severalopticalpolarizationcontrol-lers(PCs)shouldbeemployed.

ThereasonthatEOPMisavelocity-matchedde-viceisthatthephaseimposedonthecopropagatinglight(theuppersideband)isproportionaltotheap-pliedvoltageV?t?,whilethephaseofthecounter-propagatinglight(thelowersideband)ispropor-tionaltotheaveragedrivenvoltage[5–8].FortheEOPMwithtransittime?andhalf-wavevoltageV?,thephasedifference???t?isgiven

by

Fig.1.Schematicprinciplecon?gurationoftheproposedDPASK/QAMPVMsystem.©2008OpticalSocietyofAmerica

October15,2008/Vol.33,No.20/OPTICSLETTERS2333

???t?=????

c?t???s?t?=?

V?t?V1?

sin??

?

??

?

,?2?

where?istheangularfrequencyofthedrivenvolt-age.ThephasevariationbetweenthetwosidebandsisapproximatelyproportionaltothedrivenvoltageofEOPM,V?t?.Atthereceiverside,bybeatingthetwoopticalsidebands,themultilevelmm-wavesignalis?nallyobtained:

iPD??E?t???A2?t?cos?2?RFt+?c?t???s?t??

=A2?t?cos?2?RFt+???t??.

?3?

FromEq.(3),itcanbeseenthattheoutputphotocur-rentcontainsthephaseandamplitudeinformation,whichareindependentlymodulatedonEOPMandMZM2,respectively.

DAPSK,whichconveysdatabychangingboththeamplitudeandthephaseofthecarrierwave,canbeconsideredasasubclassoftheQAMmodulationfor-mat.Theconstellationofanotherso-calledcircularQAMisshowninFig.2(b).FromFig.2,itcanbeseenthatthesymbolsoftheQAMsignalwithdifferentamplitudeshaveanadditionalphaseshiftcomparedwiththatoftheDPASKsignal,whichcanbegener-atedbysimplyaddingtheamplitudeinformationinproperproportiontothephasesignaltoformthedrivensignalofEOPMinFig.1.

ForcircularQAMsignalgenerationasshowninFig.1,theoutputRFsignalafterthephotodiode(PD)canbeexpressedas

iQAM?A2

?t?cos2??t?+?A?t?

RFt+?

V?

??

?

V1?

sin????

??

??

,

?4?

where?istheattenuationcoef?cientoftheampli-tudeinformationwhenaddingtothephasesignalanddependsonthelevelofthecircularQAMsignal.ForbothDPASKandQAMformats,thephasesignalneedstobedifferentialencodedtoovercomethephaseshiftcausedbydispersionwhentransmittedover?ber.Itshouldbenoticedthatwithouttheam-plitudedatamodulation,whichmeanstodeleteMZM2inFig.1,thePVMsysteminFig.1wouldbe-comeaphotonicmm-wavephaseshifter/modulator.Figure3depictstheexperimentalsetupofthepro-posedDPASKandcircularQAMPVMsystems.Atunablelaserdiode(LD)wasemployedasthecwop-ticalsourcetomatchthecascadedopticalinterleaver.AzerochirpMZMdrivenbya12.5GHzsinusoidal

Fig.2.Constellationdiagramsof(a)4DPASKand(b)

4QAM.

Fig.3.ExperimentalsetupoftheproposedDPASK/QAMPVM

system.

clockfromamicrowavesourcewasusedtoachieveopticalcarriersuppression.Afteracascadedopticalinterleaverwith25and50GHzchannelspacing(6GHzbandwidth),thelowerandtheupperside-bandswereseparatedandinjectedintotheEOPMinoppositedirections.Withthehelpoftwoopticalcir-culators,thetwoopticalsidebandswerecombinedbya3dBOCandinjectedintoMZM2modulatedbytheamplitudesignalserial.Thephaseandamplitudese-rialsweretwoindependent2Gbits/sPRBSs,eachwithacodelengthof231?1.

Beforebeingtransmittedthrougha25kmstan-dardsingle-mode?ber(SMF),anerbium-doped?berampli?er(EDFA)wasemployedtoamplifytheopti-calsignal.Atthereceiverside,theoutputelectricalsignalfromthePDwith3dBbandwidthof70GHzwasampli?edbyamm-waveelectricalampli?erandthendownconvertedbyawidebandmixerandalow-pass?lter(LPF).Toobservetheresults,thedown-convertedsignalswereanalyzedbya40GHzoscillo-scopeanddetectedbyabiterrorrate(BER)tester.TheopticalspectrumoftheDPASKsignalbeforetransmissionisshowninFig.4.Itcanbeseenthattheuppersidebandisbroaderthanthelowerside-bandforthereasonthattheuppersidebandcarriesbothphaseandamplitudeinformation,whilethelowersidebandcarriesonlyamplitudeinformation.Thephasesignalandtheamplitudesignalwereboth2Gbits/sPRBS.TheopticalspectrumoftheQAMsignalwouldhaveasimilarshapeasinFig.4,andforsimplicity,onlytheopticalspectrumoftheDPASKsignalisgivenout.

Figure5(a)illustratestheconstellationdiagramoftheDPASKsignalintheproposedsysteminFig.2.InFigs.5(b)and5(c),thedownconvertedeyedia-gramsofaback-to-back(B-T-B)systemand

after

Fig.4.(Coloronline)OpticalspectrumoftheDPASK

signal.

2334OPTICSLETTERS/Vol.33,No.20/October15,2008

Fig.5.(Coloronline)(a)ConstellationdiagramofDPASKsignal;(b)and(c)eyediagramsoftheDPASKsignalbeforeandafter25kmSMFtransmission.

25kmSMFtransmissionindicateagoodeyeopen-ing.Themaximumqualityfactor(Qfactor)estimatedbysoftwareofthetop,middle,andbottomeyesare3.15,3.22,3.03(B-T-Bsystem)and3.11,3.19,2.98(after25kmtransmission),respectively.

TheBERcurvefortheIcomponentandtheQcom-ponentoftheQAMsignalafterdownconversionver-susreceivedopticalpowerisreportedinFig.6tostudythetransmissionperformanceoftheproposedcircularQAMsystem.Aftertransmissionoverthe25kmSMFwithoutdispersioncompensation,thepowerpenaltyoftheQcomponentisabout1.5dB.ThereasonthatthemodulationdepthoftheIcom-ponentislowerthantheQcomponent,asinFig.2(b),isthatthepowerpenaltyoftheIcomponentisabout2dB.ThedownconvertedI-componentand

Fig.6.(Coloronline)BERmeasurement.(a)and(c)DownconvertedeyediagramsoftheQ-componentinB-T-Bsystemsandafter25kmtransmission;(b)and(d)down-convertedeyediagramsoftheI-componentinB-T-Bsys-temsandafter25km

transmission.

Q-componenteyediagramsforB-T-Band25kmSMFtransmissionarealsoinsertedinFig.6.

TwonovelphotonicapproachesforDPASKandcir-cularQAMsignalgenerationareproposed.Theproof-of-conceptexperimentofthisconceptisalsore-ported,showingthegenerationand25kmSMFtransmissionofupto4Gbits/http://wendang.chazidian.comparedwithconventionalsystems,theproposedschemessigni?-cantlyreducethehardwarerequirementsbyemploy-ingthePVMtechniqueandcanbeappliedinthefu-turetosuperwidebandoptical-wirelessnetworks.ThisworkwaspartiallysupportedbytheNational863ProgramofChina(2007AA01Z264and2006AA01Z256),theInternationalCooperationPro-gram(2008DFA11670),theNationalNaturalScienceFoundationofChina(NNSFC)(60702006,60736002,and60837004),theNewCenturyExcellentTalentProjectinMinistryofEducationofChina(NCET-06-0093),(PCSIRT,ProgramforChangjiangScholarsandInnovativeResearchTeaminUniversity)(IRT0609),andthe111Project(B07005).References

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